Highly selective and reconfigurable microwave filters are of great importance in radiofrequency signal processing. Microwave photonic (MWP) filters are of particular interest, as they offer flexible reconfiguration and an order of magnitude higher frequency tuning range than electronic filters. However, all MWP filters to date have been limited by trade-offs between key parameters such as tuning range, resolution, and suppression. This problem is exacerbated in the case of integrated MWP filters, blocking the path to compact, high performance filters. Here we show the first chip-based MWP band-stop filter with ultra-high suppression, high resolution in the MHz range, and 0-30 GHz frequency tuning. This record performance was achieved using an ultra-low Brillouin gain from a compact photonic chip and a novel approach of optical resonance-assisted RF signal cancellation. The results point to new ways of creating energy-efficient and reconfigurable integrated MWP signal processors for wireless communications and defence applications.The explosive growth in mobile communications demands radio-frequency (RF) technologies with exceptional spectral efficiency such as cognitive radios, which can adapt their frequencies to exploit the available spectrum in real-time [1,2]. Such frequency-agile systems will benefit hugely from RF filters that can be tuned over many gigahertz whilst keeping high MHz-scale resolution and high selectivity to prevent severe interference due to spectrumsharing. While this is difficult to achieve with all-electronic filters [3][4][5][6][7], integrated microwave photonic (IMWP) filters [8] can readily achieve multi-gigahertz tuning range without significant degradation in their frequency response. However, these filters typically exhibit limited resolution (GHz instead of MHz linewidths) and are plagued by trade-offs between key parameters, such as between the frequency tuning range and the resolution for multi-tap filters [9][10][11][12][13]; or between the peak rejection and the resolution for resonator-based filters [14][15][16][17][18].Stimulated Brillouin scattering (SBS) [19][20][21][22] offers a route to MHz-resolution IMWP filters. Although SBS has been widely studied in optical fibers, recently there has been a growing interest in harnessing SBS in nanophotonic waveguides [22][23][24][25][26][27]. The ability to control the coherent interaction of photons and acoustic phonons in chip-sized devices (as opposed to in optical fibers many kilometres long) promises not only fascinating new physical insights, but also opens the path to realising key technologies on-chip including slow light [28,29]; narrow linewidth lasers [30]; optical frequency combs [31,32]; RF signal processing [33][34][35] and filtering [36][37][38][39][40]. In particular, SBS filters can exhibit linewidths of the order of 10-100 MHz. Such a high resolution is unmatched by most on-chip devices because it requires extremely low material losses and impractically-large devices [41].Although IMWP filters exploiting SBS on ch...

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Spectrum analysis is a key functionality in modern radio frequency (RF) systems. In particular, fast and accurate estimation of multiple unknown RF signal frequencies over a wide measurement range is crucial in defense applications. Although photonic techniques benefit from an enhanced frequency estimation range along with reduced size and weight relative to their RF counterparts, they have been limited by a fundamental trade-off between measurement range and accuracy. Here, we circumvent this trade-off by harnessing the photon and phonon interactions in a photonic chip through stimulated Brillouin scattering, resulting in an accurate estimation of multiple RFs of up to 38 GHz with a record-low error of 1 MHz.

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We demonstrate the first functional signal processing device based on stimulated Brillouin scattering in a silicon nanowire. We use only 1 dB of on-chip SBS gain to create an RF photonic notch filter with 48 dB of suppression, 98 MHz linewidth, and 6 GHz frequency tuning. This device has potential applications in on-chip microwave signal processing and establishes the foundation for the first CMOS-compatible high performance RF photonic filter.Brillouin scattering is a light-sound interaction process that occurs when photons are scattered from a medium by induced acoustic waves. 1 Stimulated Brillouin scattering (SBS) is the strongest nonlinear process and manifest optically in ultra-narrow resonances, which have been harnessed in optical fibers for slow light, sensing, and laser applications.

Instantaneous frequency measurement (IFM) of microwave signals is a fundamental functionality for applications ranging from electronic warfare to biomedical technology. Photonic techniques, and nonlinear optical interactions in particular, have the potential to broaden the frequency measurement range beyond the limits of electronic IFM systems. The key lies in efficiently harnessing optical mixing in an integrated nonlinear platform, with low losses. In this work, we exploit the low loss of a 35 cm long, thick silicon waveguide, to efficiently harness Kerr nonlinearity, and demonstrate the first on-chip four-wave mixing (FWM) based IFM system. We achieve a large 40 GHz measurement bandwidth and record-low measurement error. Finally, we discuss the future prospect of integrating the whole IFM system on a silicon chip to enable the first reconfigurable, broadband IFM receiver with low-latency.

We present the first microwave photonic phase shifter using stimulated Brillouin scattering (SBS) on-chip. The unique ability of SBS to generate both narrowband gain and loss resonances allows us to achieve low ±1.5 dB amplitude fluctuations, which is a record for integrated devices, along with 240° continuously tunable phase shift. Contrary to previous SBS-based approaches, the phase shift tuning mechanism relies on tuning the power, not the frequency, of two SBS pumps, making it more suited to on-chip implementations. We finally demonstrate that SBS pump depletion leads to amplitude response fluctuations, as well as increasing the insertion loss of the phase shifter. Advantageously, shorter integrated platforms possess higher pump depletion thresholds compared to long fibers, thus offering greater potential for reducing the insertion loss.

Optical modulation plays arguably the utmost important role in microwave photonic (MWP) systems. Precise synthesis of modulated optical spectra dictates virtually all aspects of MWP system quality including loss, noise figure, linearity, and the types of functionality that can be executed. But for such a critical function, the versatility to generate and transform analog optical modulation is severely lacking, blocking the pathways to truly unique MWP functions including ultra-linear links and low-loss high rejection filters. Here we demonstrate versatile RF photonic spectrum synthesis in an all-integrated silicon photonic circuit, enabling electrically-tailorable universal analog modulation transformation. We show a series of unprecedented RF filtering experiments through monolithic integration of the spectrum-synthesis circuit with a network of reconfigurable ring resonators.